Steel part having high strength and high bendability

Abstract

Steel part having a chemical composition comprising, by weight %, 0.18≤C≤0.27, 0.18≤Si≤0.30, 1.0≤Mn≤1.5, 0.14≤Cr≤0.25, 0.02≤Al≤0.06, 0.02≤Ti≤0.06, 0.0020≤B≤0.0040, 0≤S≤0.008, 0≤N≤0.020, the remainder of the composition being iron and unavoidable impurities resulting from the elaboration process, having a microstructure including, in surface fraction, 95% or more of martensite and a prior austenite grain skin refinement ratio equal to or above 1.2, the ratio being defined as PAGS_B/PAGS_S, wherein PAGS_B is the average prior austenite grain diameter in the bulk ( 3 ) expressed in μm and PAGS_S is the average prior austenite grain diameter in the skin ( 2 ), also expressed in μm.

What is claimed is: 
     
         1 . A steel part having a chemical composition comprising, by weight %: 
       
         
           
             
               0.18 
               ≤ 
               C 
               ≤ 
               0.27 
             
           
         
         
           
             
               0.18 
               ≤ 
               Si 
               ≤ 
               0.3 
             
           
         
         
           
             
               1. 
               ≤ 
               Mn 
               ≤ 
               1.5 
             
           
         
         
           
             
               0.14 
               ≤ 
               Cr 
               ≤ 
               0.25 
             
           
         
         
           
             
               0.02 
               ≤ 
               Al 
               ≤ 
               0.06 
             
           
         
         
           
             
               0.02 
               ≤ 
               Ti 
               ≤ 
               0.06 
             
           
         
         
           
             
               0.0015 
               ≤ 
               B 
               ≤ 
               0.004 
             
           
         
         
           
             
               0 
               ≤ 
               S 
               ≤ 
               0.008 
             
           
         
         
           
             
               0 
               ≤ 
               P 
               ≤ 
               0.04 
             
           
         
         
           
             
               0 
               ≤ 
               N 
               ≤ 
               0.02 
             
           
         
         
           
             
               0 
               ≤ 
               Mo 
               ≤ 
               0.3 
             
           
         
         
           
             
               0 
               ≤ 
               Nb 
               ≤ 
               0.1 
             
           
         
         
           
             
               0 
               ≤ 
               V 
               ≤ 
               0.3 
             
           
         
         
           
             
               0 
               ≤ 
               Cu 
               ≤ 
               0.25 
             
           
         
         
           
             
               0 
               ≤ 
               Ni 
               ≤ 
               0.25 
             
           
         
         
           
             
               0 
               ≤ 
               Sn 
               ≤ 
               0.05 
             
           
         
         
           
             
               0 
               ≤ 
               As 
               ≤ 
               0.03 
             
           
         
         
           
             
               0 
               ≤ 
               Sb 
               ≤ 
               0.03 
             
           
         
         
           
             
               0 
               ≤ 
               Pb 
               ≤ 
               0.03 
             
           
         
         a remainder of the composition being iron and unavoidable impurities resulting from processing; and 
         a microstructure comprising, in surface fraction, 95% or more of martensite, 0-5% bainite, 0-5% ferrite and 0-5% retained austenite, 
         the steel part comprising a bulk portion occupying a centermost 80% of its thickness and a skin occupying an outermost 10% of the thickness on either side of the bulk portion, 
         a prior austenite grain skin refinement ratio equal to or above 1.2, the prior austenite grain skin refinement ratio being defined as PAGS_B/PAGS_S, wherein PAGS_B is the average prior austenite grain diameter in the bulk portion expressed in μm and PAGS_S is the average prior austenite grain diameter in the skin, also expressed in μm. 
       
     
     
         2 . The steel part according to  claim 1  wherein the average prior austenite grain diameter in the skin PAGS_S is equal to or lower than 6.0 μm. 
     
     
         3 . The steel part according to  claim 1  wherein the steel part was obtained by hot stamping a steel blank. 
     
     
         4 . The steel part according to  claim 1  wherein the steel part has an ultimate tensile strength measured in the transverse direction equal to or higher than 1350 MPa. 
     
     
         5 . The steel part according to  claim 1  wherein the steel part has a bending angle α1.5 normalized to 1.5 mm thickness and measured in the transverse direction equal to or higher than 57°. 
     
     
         6 . The steel part according to  claim 1  wherein the steel part is topped with a metallic coating comprising an aluminum content, expressed in weight %, equal to or higher than 50%. 
     
     
         7 . A process for producing a steel part comprising the following steps:
 casting a semi-product at a casting speed equal to or greater than 3.0 m/min, the semi-product having a composition   
       comprising, by weight %:
 0.18≤C≤0.27 
 0.18≤Si≤0.30 
 1.0≤Mn≤1.5 
 0.14≤Cr≤0.25 
 0.02≤Al≤0.06 
 0.02≤Ti≤0.06 
 0.0015≤B≤0.0040 
 0≤S≤0.008 
 0≤P≤0.04 
 0≤N≤0.020 
 0≤Mo≤0.3 
 0≤Nb≤0.1 
 0≤V≤0.3 
 0≤Cu≤0.25 
 0≤Ni≤0.25 
 0≤Sn≤0.05 
 0≤As≤0.03 
 0≤Sb≤0.03 
 0≤Pb≤0.03 
 a remainder of the composition being iron and unavoidable impurities resulting from processing; 
 optionally cooling the semi-product after casting, 
 maintaining or reheating the semi-product in a temperature range of 1075° C. to 1200° C.; 
 hot-rolling the semi-product using a finishing temperature equal to or above 850° C. and a coiling temperature equal to or above 525° C. to obtain a hot rolled steel sheet; 
 pickling the hot rolled steel sheet; 
 optionally cold-rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; 
 applying an annealing step to the hot rolled or cold rolled steel sheet by heating in an annealing furnace, to an annealing temperature TA, expressed in ° C., comprised from 700° C. to 850° C., the furnace further comprising a soaking section where the steel sheet is maintained at the temperature TA for a holding time tA, comprised from 10 seconds to 20 minutes; 
 optionally cooling the annealed steel sheet to a temperature range from 400° C. to 700° C.; 
 optionally coating the annealed steel sheet with a metallic coating; 
 cutting the steel sheet to a predetermined shape, so as to obtain a steel blank; 
 heating the steel blank to a temperature equal to or above Ac3 and equal to or lower than 950° C. during 10 seconds to 15 minutes to obtain a heated steel blank; 
 transferring the heated steel blank to a forming press; 
 hot-forming the heated blank in the forming press to obtain a formed part; 
 die-quenching the formed part to obtain a steel part; and 
 optionally paint baking said steel part at a temperature from 150° C. to 250° C. for a time comprised from 10 minutes to 2 hours.